At present, global warming is considered an urgent issue and one of the most important tasks for humanity to deal, with as it could endanger the existence of humans. Carbon dioxide, a large quantity of which has been accumulated in the world due to the use of fossil fuels, accounts for much of the global warming phenomenon. Further, the rate of carbon dioxide emission has recently increased exponentially, and numerous intensive studies have been conducted all over the world to develop techniques for dealing with this issue.
Examples of conventional techniques for hydrogenating carbon dioxide in order to fix carbon dioxide include photoelectrochemical techniques utilizing natural energy, such as sunlight, and biochemical techniques utilizing microorganisms. Disadvantageously, the efficiency of these techniques is low, and the rate of processing is much lower than that of carbon dioxide emission. The reduction of carbon dioxide with the use of a catalyst was the only conversion technique having a sufficient rate of processing. This procedure, however, requires a specific reaction vessel in which hydrogen and carbon dioxide can be simultaneously subjected to catalytic reactions in the presence of an adequate catalyst at sufficiently high temperature and high pressure. Furthermore, a relatively low-temperature synthetic catalyst, for example, a methanol synthetic catalyst that causes reactions at 250° C. to 300° C., mainly composed of a mixed oxide of zinc and copper, must be first prepared, accommodated in a reaction vessel, subjected to reactions, and replaced with another catalyst at any time. Accordingly, this procedure must be intermittently carried out, which prevents the processing of emitted carbon dioxide in large quantities. There exits many literatures concerning the hydrogenation of carbon dioxide, although conventional techniques suffer from serious drawbacks as described below.
The chemical formula of hydrogenating carbon dioxide is shown as below.CO2+3H2→CH3OH+H2O−14.3 kcal/mol   [Chemical Formula 1]
(The symbol “-” denotes heat generation. The same applies hereinafter.)
According to this chemical formula, the conversion rate at chemical equilibrium cannot be exceeded due to thermodynamic restrictions. It is highly insufficient to conduct the above reaction using a main catalyst of zinc or copper oxide since the conversion rate is as low as approximately 40% under reaction conditions of approximately 200° C. to 250° C. at 50 atm to 100 atm, and many substances remain unreacted. In this case, therefore, unreacted substances must be recycled by being separated after the reaction and then subjected to reactions again, or a selective permeable membrane must be used to separate the product from unreacted gas. Thus, conventional techniques are not yet sufficient due to the necessity for considerably complicated equipment, as well as the existence of technical and practical difficulties.